Note: Descriptions are shown in the official language in which they were submitted.
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.-- . I
MODE FIELD DIAMETER CONVERSION FIBER,
METHOD FOR LOCALLY CHANGING THE REFRACTIVE
INDEX OF OPTICAL WAVEGUIDES AND METHOD FOR
s FABRICATING OPTICAL WAVEGUIDE PREFORMS
1. Field of the Invention
The present invention relates to fiber optics.
2. Background of the Invention
A conventional mode field diameter conversion fiber
comprising a length of an optical waveguide including a quartz
glass cladding and a germanium-doped quartz glass core, wherein
the core diameter varies along the optical waveguide, increasing
towards its end, is disclosed, e.g., by K.Shiraishi, Y.Aizava,
S.Kawakami in the reference titled as "Beam Expanding Fiber
Using Thermal Diffusion of Dopant" in IEEE Journal of
Lightwave Technology, 1990, vol.8, No.8, p.l 151-1161. In the
mode field diameter conversion fiber of the above reference, the
longitudinal variation in a core diameter is provided owing to
redistribution of the radial doping profile, which forms the
refractive index structure of the optical waveguide, in the process
2s of thermal diffusion of germanium.
The probl~m with the mode field diameter conversion fiber is
a complicated technology caused by a small coefficient of
germanium diffusion to quartz glass, resulting in a prolonged heat
treatment of the optical waveguide, required to produce a mode
field diameter conversion fiber. Apart from that, the diffusion
occurs efficiently only at the temperature of 1600~C to 1800~C that
is close to the melting point and, therefore, causes deformations in
optical waveguides.
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Closely approaching the claimed invention is a mode field
diameter conversion fiber comprising a length of an optical
waveguide including a quartz glass cladding and a doped quartz
glass core, wherein the core diameter varies along the optical
s waveguide, increasing towards its end (see, e.g., US Patent
5381503, Int.CI. G 02 B 6/10). In the prior art fiber, the core is
initially doped with germanium and fluorine. Unlike germanium,
fluorine reduces the quartz glass refractive index and additionally
exhibits a greater thermal diffusion coefficient at the temperature
o of 1 600~C to 1 800~C. As the result, when the optical waveguide
with the double-doped core is heated, fluorine faster penetrates
into the cladding, providing an efficient increase in the glass core
refractive index and, therefore, a decreased mode field diameter.
A disadvantage of the above prior art is a complicated
15 fabrication of the mode field diameter conversion fiber and a
narrow range of mode field diameter variation, which is caused by
a limited concentration of fluorine that can be introduced into the
core along with germanium. Furthermore, the fluorine diffusion
will unavoidably lead to the appearance, in such an optical
waveguide, of regions with a decreased refractive index, which
hampers coupling to conventional optical waveguides.
Described by O.Hill, Y.Fujii, D.C.Johnson and B.S.Kawasaki
in the reference titled as "Photosensitivity in Optical Fiber
Waveguides: Application to Reflection Filter Fabrication".
Appl.Phys.Lett. 1978, Vol.32, No. 10, p.647-649, is a method for
locally changing the refractive index of an optical waveguide,
involving subjecting the optical waveguide to external exposure.
Change in the refractive index occurs due to the photorefractive
effect and is caused by the presence of defects that give rise to the
30 appearance of specific bands in the absorption spectrum of the
optical waveguide core. In this case, a two-photon interaction
takes place. In the optical waveguide, an incoming beam and a
beam reflected from the end face interfere, causing a periodic
change in the refractive index.
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Disadvantages of the above prior art method include a
sophisticated process, a small change (~ ~ 10-6) in the refractive
index, impossibility to vary a period of the grating produced, high
cost and complexity of operating the apparatus implementing the
method.
Another method for locally changing the refractive index of
an optical waveguide, involving subjecting the optical waveguide
to external exposure, was disclosed by G.Meltz, W.W.Morey,
W.H.Glen in the reference titled as "Formation of Bragg Gratings
lo in Optical Fibers by a Transverse Holographic Method". Opt.
Lett., 1989, vol.14, No.15, p.823-825. In accordance with the
method, an optical waveguide is exposed to a laser radiation at a
wavelength close to 240 nm. A grating is formed in the optical
waveguide using an interference effect, by directing two beams at
lS an angle ~ to the optical waveguide surface. A period of the
grating formed can be altered by varying the angle ~.
The above prior art method, however, suffers from an
elaborated technology of locally changing the refractive index of
an optical waveguide.
K.O.Hill, F.Bilodean, B.Malo, D.C.Malo, D.C.Johnson,
I.S.Kinner, "Efficient Mode Conversion in Telecommunication
Fiber Using Externally Writing Gratings". Electron.Lett., 1990,
vol.26, p.1270-1272, disclose a method for locally changing the
refractive index of an optical waveguide, involving subjecting the
25 optical waveguide to external exposure. In this method, a
photosensitive optical waveguide is exposed, at the side surface, to
ultraviolet laser radiation that has passed through a narrow slit.
Within the exposure region, a photo-induced local increase in the
refractive index occurs, which corresponds to writing a single
30 groove of the grating. The exposure is then interrupted and the
optical waveguide is axially displaced relative to the laser beam at
a distance equal to the grating period, following which a next
grating groove is written. By periodically repeating the optical
waveguide exposure and axial displacement procedures, a
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periodical sequence of core regions having different refraceion
indices is formed, representing a grating.
However, the process of locally changing the refractive index
is rather complicated owing to use of the ultraviolet laser
s radiation and by virtue of the fact that a number of photosensitive
types of optical waveguides is limited. In addition, the employed
ultraviolet lasers are relatively expensive, sophisticated in service,
have insufficient reliability and their radiation is hazardous to
human eye.
o Closely approaching the present invention is a method for
locally changing the refractive index of an optical waveguide,
involving heating a length of an optical waveguide including a
cladding and a core, each one being based on quartz glass and at
least one of them being doped, providing a local thermal diffusion
15 of the elements contained in the core to the cladding, and/or
providing a local thermal diffusion of the elements contained in
the cladding to the core. See, e.g. US Patent No.538 1503, Int.CI.
G 02 B 6/10. In this method, a length of an optical waveguide is
heated in a resistance-heated furnace.
The double doping with germanium and fluorine significantly
complicates the fabrication of optical waveguides for a mode field
diameter conversion fiber. A further disadvantage of this method
is that it is impossible to control the law of changing the core
refractive index along the optical fiber axis, which is defined by
2s the temperature distribution within the heating apparatus and
cannot be modified in the course of the process. A further problem
is that the optical waveguide undergoes deformation when heated,
owing to relaxation of lateral stresses appearing when the optical
waveguide is fixed.
A method for fabricating a preform for a quartz glass-based
optical waveguide, including feeding a mixture of molecular
gaseous agents containing atoms of oxygen and silicon into a
substrate tube, and depositing the products of the reaction
occurring in the mixture on the inner surface of the substrate
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S
tube, is described, e.g., in the Reference Book of Fiber Optic
Communication Links, L.M.Andrushko, V.A.Voznesenskii,
V.B.Katok et al.; under edition of S.V.Svechnikov and
L.M.Andrushko, Kiev, Tekhnika, 1988, p.69.
However, fabrication of a preform from nitrogen-doped
quartz glass is unfeasible under thermodynamic equilibrium
conditions typical for this method.
A method for fabricating preforms for optical waveguides
based on nitrogen-doped quartz glass, that is most closely
10 approaching the claimed invention, includes feeding a mixture of
molecular gaseous agents containing atoms of nitrogen, oxygen
and silicon into a substrate tube, exciting a microwave discharge
in the mixture and depositing products of the reaction occurring
in the mixture on the inner surface of the substrate tube. See, e.g.,
15 E.M.Dianov, K.M.Golant, R.R.Kharpko, A.S.Kurkov, A.L.
Tomashuk. "Low-Hydrogen Silicon Oxynitride Optical Fibers
Prepared by SPCVD". IEEE Journal of Lightwave Technology,
1995, vol.l3, No.7, p.l471-1474. The basis for this method is
fabrication of an optical waveguide preform by plasma-enhanced
20 synthesis of nitrogen-doped quartz glass layers on the inner wall
of a quartz glass substrate tube by the SPCVD process.
The method, however, is technologically complicated since it
does not employ process parameters in the preform synthesis,
which would allow a reproducible control of the nitrogen
2s concentration in the quartz glass, required to obtain a desired
refractive index difference ~n between the optical waveguide core
and cladding.
3. Summary of the Invention
It is an object of the present invention to facilitate
fabrication of optical waveguides and apparatuses on their basis.
The aforementioned object is attained by a conventional
mode field diameter conversion fiber comprising a length of an
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optical waveguide including a quartz glass cladding and a doped
quartz glass core, the core diameter varying along the optical
waveguide, increasing towards its end, wherein in accordance with
the present invention the core of the optical waveguide is doped
s with a dopant, such as nitrogen, the dopant concentration being
from 0.01 at. % to 5 at. %.
The object of the invention is also attained by a
conventional method of fabricating a mode field diameter
conversion fiber, including heating a length of an optical
lo waveguide comprising a cladding and a core, each one being based
on quartz glass and at least one of them being doped, wherein in
accordance with the invention local thermal diffusion is provided
by heating the length of the optical waveguide having the core
doped with nitrogen at the concentration from 0.01 at.% to 5 at.%,
s said heating being performed by current of an electric arc or
radiation of an infrared laser.
In particular, the heating of the length of the optical
waveguide is periodically interrupted, and during the periodic
interruption the heating region is relatively displaced along the
20 optical waveguide axis at a distance divisible by the spatial
interval of the refractive index variation.
In particular, a local thermal diffusion of such dopants as
phosphorous, aluminum, sodium, potassium, lithium, cesium,
germanium, fluorine, boron and/or at least one of the rare-earth
2s elements, is provided.
The length of the optical waveguide can be heated by an
electric arc current from 5 mA to 500 mA for a time from 0.1 s to
600 min, the electric arc being located within a region as large as I
mm to 15 mm along the optical waveguide axis and generated by
30 electrodes with sharpened ends, and said heating is followed by
cutting off the length of the optical waveguide at right angle to its
axis in a region corresponding to the predetermined mode size.
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The electric arc can be displaced along the optical waveguide
axis at a variable speed, and/or the current of the electric arc can
be varied.
In particular, to avoid deformations caused by mechanical
s stresses when an optical waveguide is fixed in two points, an
auxiliary optical waveguide is spliced with the optical waveguide
before the thermal diffusion.
The heating can be provided by radiation of CO-laser, CO2-
laser, erbium laser or holmium laser.
o In particular, a length of an optical waveguide with a silicon
oxynitride (Si3N4:SiO2)-based core can be heated.
A length of an optical waveguide with a fluorine and/or
boron-doped cladding can be heated.
The object of the invention is also attained by a
conventional method for fabricating preforms for optical
waveguides based on nitrogen-doped quartz glass, including
feeding a mixture of molecular gaseous agents containing atoms of
nitrogen, oxygen and silicon into a substrate tube, exciting a
microwave discharge in the mixture, and depositing products of
the reaction occurring in the mixture on the inner surface of the
substrate tube, wherein in accordance with the invention the
molecular gaseous agents, that are fed into the substrate tube, are
mixed so that less than five atoms of oxygen fall on every silicon
atom, and more than one atom of nitrogen fall on every 1000
atoms of oxygen.
In particular, the mixture of molecular gaseous agents is at
least one of the following mixtures: SiCI4+02+N2, SiCI4+02+NO2,
SiCI4+02+N203, SiCI4+02+N204, SiCI4+02+N20, SiCI4+02+NO
and/or SiCI4+02+NH3.
In particular, the temperature of the substrate tube is from
900~C to 1300~C.
In particular, the power required to excite the microwave
discharge in the substrate tube is from 0.1 kW to 10 kW.
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In particular, the gaseous agents within the substrate tube
are under the pressure from O.OS mm Hg to S0 mm Hg.
An optical waveguide mode field diameter conversion fiber, a
method for locally changing the refractive index of an optical
s waveguide and a method for fabricating preforms for optical
waveguides, that are employed to produce the mode field diameter
conversion fiber in accordance with the invention, are united by a
single inventive concept and jointly provide the attainment of the
aforementioned technical result.
4. Brief Description of the Drawings
The present invention will be apparent from the following
description of its embodiments, taken in conjunction with the
15 accompanying drawings in which
Fig. I illustrates the structure of a mode field diameter
conversion fiber; a distribution of electric field E(r) across the
core diameter; core diameter d and refractive index n as a function
of distance z along the optical waveguide axis;
Fig.2 is a block diagram of an apparatus for fabricating a
mode field diameter conversion fiber, using an electric arc;
Fig.3 is a block diagram of an apparatus for fabricating a
mode field diameter conversion fiber, using a CO-laser;
Fig.4 is a directional diagram of the optical waveguide
25 radiation before to thermal diffusion;
Fig.S is a directional diagram of the optical waveguide
radiation after thermal diffusion;
Fig.6 is a block diagram of an apparatus for modulating the
refractive index using an electric arc;
Fig.7 is a block diagram of an apparatus for modulating the
refractive index using a laser;
Fig.8 illustrates a characteristic transmission spectrum of a
long-period grating 20 mm in length, produced using thermal
diffusion;
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Fig.9 is a block diagram of an apparatus implementing the
method for fabricating preforms in accordance with the present
invention.
5. Detailed Description of the Embodiments
Referring to Fig. I, a mode field diameter conversion fiber
comprises a length of an optical waveguide I including a cladding
2 and a core 3, wherein a diameter d of the core 3 is increasing
o towards the end 4 of the length of the optical waveguide, while the
effective refractive index n thereof is decreasing.
In an apparatus for fabricating a mode field diameter
conversion fiber, using an electric arc (Fig.2), electrodes 5 with
sharpened ends are arranged at right angle to the axis of the
15 length of the optical waveguide 1. The optical waveguide I is fixed
by a clamp 6. An auxiliary optical waveguide 7 is secured in a
three-coordinate micrometer positioner 8 with a clamp 9.
An electric arc 10 is displaced along the optical waveguide
axis by a translator 1 1 with a step motor 12. The electrodes are
20 connected to a power source 13 that sets the arc current. The
electric arc displacement and current are controlled by a computer
14.
In an apparatus for fabricating a mode field diameter
conversion fiber, using a CO-laser 15 (Fig.3), a radiation beam 16
2s is focused at an optical waveguide 1 by a lens 17. A power
regulator 18 modifies intensity of the laser radiation 16. A
translator 19 with a step motor 12 displaces the optical waveguide
1 along its axis relative to the radiation beam 16. The power
regulator 18 and motion of the translator 19 are controlled by a
30 computer 14.
To determine the mode size, a radiation pattern is measured
at the output of the mode field diameter conversion fiber (Figs.4
and 5).
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An apparatus (Fig.2) for implementing the method according
to the present invention operates as follows.
An optical waveguide 1 is spliced by an electric arc 10 with
an auxiliary optical waveguide 7 that has been preliminary
s adjusted by a micrometer positioner 8. The arc 10 is then located
at the length of the optical waveguide I in which thermal diffusion
is conducted, or the arc is displaced along the optical waveguide.
With stationary electrodes 5, the size of the heating region is as
large as 0.1 mm to 5 mm, depending on distances between the
10 electrodes 5 and the arc 10 current. The exposure time of a local
region of the optical waveguide I to the electric arc 10 depends on
the displacement speed of the electrodes S along the optical
waveguide 1. By adjusting the displacement speed of the arc 10,
one can tailor the law of changing the core 3 diameter along the
optical waveguide 1 as its end 4 approaches. Once the thermal
diffusion is over, the optical waveguide I is cut off at right angle
in the region corresponding to the required mode size, close to the
area where the optical waveguide 1 is spliced with the auxiliary
optical waveguide 7.
An alternative apparatus (Fig.3) for implementing a method
in accordance with the present invention operates as follows.
An optical waveguide I is spliced by a laser radiation beam
16 to an auxiliary optical waveguide 7 that has been preliminarily
adjusted by a micrometer positioner 8. The optical waveguide I
2s and the auxiliary optical waveguide 7 are then together displaced
by a translator 19 along their axis relative to the laser radiation
beam 16. The exposure time of a local region of the optical
waveguide I to a CO-laser radiation depends on the displacement
speed of the optical waveguide I relative to the laser beam 16. By
30 adjusting the displacement speed of the optical waveguide 1, one
can tailor the law of changing the core 3 diameter along the
optical waveguide I as its end 4 approaches. The law of changing
the core 3 diameter along the axis of the optical waveguide 1 can
be also set by controlling the laser radiation intensity, using a
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11
power regulator 18, at a uniform displacement of the optical
waveguide I relative to the laser beam 16. With a stationary
optical waveguide, a size of the heating region can be as large as
0.01 mm to 15 mm depending on the focal length of the lens 17
s and its position. Once the thermal diffusion is over, the optical
waveguide 1 is cut off at right angle to the axis, in the region
corresponding to the required mode size, close to the area of
splicing with the auxiliary optical waveguide 7.
To avoid a core-to-cladding mode conversion, length L of
10 the optical waveguide, in which the mode diameter is being varied,
and core diameter d should be related as L~>d. In practice, this is
provided if L > lmm. Such a smooth change in the core diameter
is attained when a stationary electric arc is located within an area
as large as 1 mm to 5 mm along the optical waveguide axis, both
ls the distance between the sharpened ends of the electrodes and the
current of the electric arc 10 being varied, or when the arc is
displaced along the optical waveguide axis so that to heat the
optical waveguide of length from I mm to 15 mm. The
temperature, to which the length of the optical waveguide 1 can be
20 heated, also depends on the distance between the electrodes and
the arc current. Our experience suggests that depending on a
dopant type, the electric arc current should be from 5 mA to 500
mA. To provide the required thermal diffusion, the exposure time
of the optical waveguide I to the arc 10 should be from 0.1 s to
2s 600 min, and the displacement speed of the arc along the optical
waveguide axis can be varied between 1 llm/s and 1 cm/s.
Coupling of a mode field diameter conversion fiber to the
other fiber elements is much easier owing to the fact that the
refractive index profile of the mode field diameter conversion
30 fiber does not include regions with a decreased refractive index,
which are unavoidable when one of the dopants is fluorine. Our
experience led us to conclude that the nitrogen concentration
should be from 0.01 at. % to 5 at. %.
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12
In an apparatus for a periodic modulation of the refractive
index, using an electric arc (Fig.6), electrodes 5 are arranged at
right angle to the axis of the optical waveguide 1 including a
cladding 2 and a core 3. The optical waveguide is fixed by clamps
5 6 and 9. Electrodes are connected to a power source 13 that sets an
electric arc 1 0 current. A translator 11 with a step motor 1 2,
adapted to displace electrodes 5 along the optical waveguide axis,
forms periodic local areas 20 in which the core refractive index,
due to the thermal diffusion, differs from that at the areas that
lo were not subjected to the thermal diffusion, as shown in the plot
of Fig.6 illustrating core refractive index n as a function of
coordinate z along the optical waveguide axis. To monitor the
refractive index modulation, an apparatus (Fig.6) comprises a
white light source 21 and an optical spectrum analyzer 22. A
15 computer 1 4 controls the step motor 1 2 of the translator 11 and
defines a condition of the electric arc 10 by controlling the power
source 1 3.
In an apparatus (Fig.7) for modulating the refractive index
using a laser 15, a laser radiation 16 is focused by a lens 17 at a
20 core 3 of an optical waveguide 1. The exposure time is set by a
shutter 23, and the optical waveguide displacement relative to the
laser beam 16 is provided by a translator 19 with a step motor 12
under control of a computer 14.
The basis for the present invention is a novel mechanism of
25 modulating the refractive index of a core 3 of an optical
waveguide 1. Whereas in the closest prior art the refractive index
changes owing to the photorefractive effect under exposure to
ultraviolet radiation, in accordance with the present invention this
is attained owing to thermal diffusion of elements contained in a
30 core 3 and/or cladding 2, when a local region of the optical
waveguide 1 is heated by an electric arc discharge 10 (Fig.6) or by
a radiation 16 of an infrared laser 15 (Fig.7). The diffusion results
in a local change in the core 3 refractive index, i.e. in writing one
groove of a grating. The entire grating is formed by successively
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13
writing individual grooves as the heating region is displaced along
the axis of the optical waveguide I at a distance divisible by the
period of the grating. The grating grooves can be written by
successively displacing the optical waveguide I relative to the
s heating source 10 (Fig.6) or 16 (Fig.7) or vice versa, at one period
of the grating, at two periods, by double passage across the same
region of the optical waveguide 1, etc.
Temperature and time of writing the grating grooves depend
on chemical compositions of the core 3 and the cladding 2 of the
lo optical waveguide 1, and can be easily determined through
conventional experiments. In quartz optical waveguides I with a
GeO2-doped core 3, the diffusion of Ge efficiently occurs at the
temperatures above 1 400~C. A slow diffusion results in a long
writing time, and a high temperature, close to the SiO2 melting
15 point, can cause deformations in the optical waveguide 1.
To this drawback are not subjected, in particular, quartz
optical waveguides I with a core 3 based on silicon oxynitride
Si3N4: SiO2, or optical waveguides I with a quartz core 3 and a
fluorine-doped cladding 2, as diffusion coefficients of N and F to
20 SiO2 are substantially superior to that of Ge. As the result, their
diffusion occurs efficiently at significantly lower temperatures,
and one groove is written in a considerably less time.
An optical waveguide I is locally heated by an electric arc
discharge 10 which is similar to that employed in apparatuses for
2s splicing optical waveguides. A heating area length and a
temperature can be varied through the selection of a distance
between electrodes 5 and the arc 10 current. In particular, the
distance between electrodes 5 can be in the range from D to 100 D
and greater, where D is the external diameter of an optical
waveguide 1, and the current can be in the range from 5 mA to 50
mA and above.
The local heating can be also implemented by a laser 15 with
a radiation wavelength 16 at which a material of the optical
waveguide I exhibits a considerable absorption of the order of
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' 14
0.1 cm-' to 10 cm-' . To this end, infrared lasers can be employed,
such as a CO-laser with a wavelength of about 5 ~lm, C02-laser
with a wavelength of about 10 ~m, erbium laser with a wavelength
of 2.76 llm, and holmium laser with a wavelength of 2.94 ,um.
s Although the absorption in a pure quartz glass at the erbium laser
radiation wavelength is less than 5 x 10-2 cm-', it can be raised by
several orders of magnitude owing to an OH-bond oscillation
absorption line in a quartz optical waveguide (wavelength of 2.76
~lm) containing water.
With the electric arc 10, the minimum heated region that can
be attained is approximately equal to the external diameter of the
optical waveguide 1, generally 125 llm. The exposure to infrared
laser radiation can provide the minimum heated region under 10
m .
In long-period gratings, owing to a resonant coupling
between the modes of the core 3 and the cladding 2, energy is
selectively coupled from the core 3 mode to the cladding 2 mode
and rapidly dissipates in the external polymer coating applied
onto the cladding 2. These resonant wavelengths are defined by a
grating period and range, in general, from 200 ~lm to 400 ~lm. It is
precisely this effect that proves the existence of modulation in the
refractive index of the optical waveguide 1.
When compared to the closest prior art, the present
invention provides the following advantages.
Thermal diffusion allows the refractive index to be
modulated even in the optical waveguides 1 exhibiting a weak
photorefractive effect.
The invention employs an arc electric discharge 10 or an
infrared laser 15 as a heating source that is simple both in
production and operation and, therefore, relatively cost effective.
A radiation 16 of an infrared laser 15 and of an electric arc
10 is more safe for a human than a radiation of an ultraviolet
I as er .
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With an electric arc 10 (Fig.6), a method according to the
invention is implemented as follows.
An optical waveguide 1, from which a protective polymer
coating has been stripped, is fixed in stationary clamps 6 and 9,
and then electrodes 5 are set by a micrometer positioner 8 and a
step motor 12 along axis z of the optical waveguide 1 in a position
corresponding to a first region of changing the refractive index,
whereupon a power source 13 of the electric arc 10 is turned on
and the optical waveguide I is heated during a predetermined
0 time, following which the power source 13 of the electric arc 10 is
turned off. The electrodes 5 are next displaced by the translator 11
with a step motor 12 along the axis z of the optical waveguide 1 to
a next region of changing the refractive index, spaced apart from
the first region at the refractive index modulation period,
following which the power source 13 of the electric arc 10 is
turned on again and the optical waveguide 1 is heated during a
predetermined time, whereupon the power source 13 of the arc 10
is turned off. The above operations are periodically repeated so
that a number of the repetitions corresponds to the required
number of the refractive index modulation periods in the core 3 of
the optical waveguide 1. The step motor 12 of the translator 1 1
and the power source 12 of the electric arc 10 are controlled by a
computer 14 to provide the desired sequence and periodicity of
operations when following an appropriate routine. To monitor the
process of forming a refractive index grating, a transmission
spectrum of the optical waveguide I being heated is measured by
an optical spectrum analyzer 22 and a white light source 21. Once
the required depth of absorption peaks corresponding to a
resonant interaction between the modes of the core 3 and the
30 cladding 2 has been attained, the process can be terminated.
As opposed to the apparatus of Fig.6, in an apparatus for
periodic modulation of the refractive index, using an infrared
laser 15 (Fig.7), an optical waveguide 1 is fixed in clamps 6 and 9
secured on a movable part of the translator 19 which provides a
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16
displacement of the optical waveguide I relative to a radiation
beam 16 of an infrared laser 15 performing a local heating of the
optical waveguide 1. The beam is focused by a lens 17 and
periodically interrupted by a shutter 15. The apparatus of Fig.7
s operates according to a routine similar to that has been described
above for the apparatus of Fig.6 and realized by the computer 14.
To illustrate an apparatus for implementing a method for
fabricating preforms for optical waveguides in accordance with the
present invention, Fig.9 depicts a substrate quartz tube 24, a
lo mixture 25 of molecular gaseous agents, a deposition area 26, a
microwave power 27 applied through a waveguide 28 to an annular
gap 29, a surface plasma wave 30, a matching unit 31 with a piston
32 and a plasma column 33.
The mixture of molecular gaseous agents 25 of the
15 composition SiCI4 + 02 + N2 is supplied to a quartz glass substrate
tube 24 under a pressure of several mm Hg. From the opposite
side, a microwave power 27 is applied to the tube 24, which
maintains a stationary discharge in the mixture, generating the
plasma column 33. The agents 25 are dried oxygen and nitrogen
20 along with silicon tetrachloride. Entering the region of the plasma
column 33, the agents 25 undergo chemical transformations owing
to appearance, in the mixture, of active radicals which are
"generated" in the mixture from unexcited molecules when they
interact with plasma "hot electrons". As the result, silicon
2s tetrachloride transforms to silicon oxide which is adsorbed by the
walls of the quartz tube 24 and is finally oxidized to silicon
dioxide, owing to a heterogeneous reaction involving nitrogen-
containing radicals, so that a doped quartz glass deposition area
26 is formed. By variation of a length of the plasma column 33
30 through altering the applied microwave power 27, the deposition
area 26 can be scanned along the substrate quartz tube 24 and, in
so doing, glass can be deposited layer-by-layer onto its inner
surface .
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17
The thermal diffusion coefficient of nitrogen to quartz glass
is much greater than that of germanium. This facilitates
fabrication of a mode field diameter conversion fiber since, unlike
the closest prior art, there is no necessity to add an additional
dopant with a high thermal diffusion coefficient for producing an
expanding core 3 (Fig. I) and, furthermore, the time of thermal
diffusion decreases.
Once the deposition process has been finished, the substrate
quartz tube 24 with a layer of nitrogen-doped glass deposited onto
10 the inner surface is collapsed into a rod through heating in a
burner flame. The so-produced preform is then drawn into a
single-mode optical waveguide 1.
A quartz optical waveguide 1 with a nitrogen-doped core 3
was used to fabricate a mode field diameter conversion fiber
ls increasing a mode diameter approximately by an order of
magnitude. Nitrogen concentration in the core 3 was I at. %. The
diameter of the core 3 of the initial optical waveguide I was 4.5
~lm, the refractive index difference between the core 3 and the
cladding 2 was ~n = 0.012. The region heated by a stationary arc
20 was as large as 0.3 mm, the electric arc current was 14 mA, the
length of the heated area of the optical waveguide was 5mm. The
arc displacement speed was gradually varied from 500 ,um/s to 10
~lm/s as approaching the splicing point between the optical
waveguide I and the auxiliary fiber 7. The exposure time was I
25 min. During the thermal diffusion no deformations in the optical
waveguide was revealed, since the optical waveguide I was secured
only at one point by a clamp 6 and spliced at the other side with
the auxiliary fiber 7 that was preliminary adjusted by a
micrometer positioner 8 (Fig.2).
A mode diameter was calculated from a pattern of radiation
leaving the optical waveguide, that was illuminated from a laser
diode at the opposite end face. Fig.4 shows a radiation pattern of
the optical waveguide I before the thermal diffusion. It can be
seen that the width of the radiation pattern across one-half
CA 02242842 1998-07-10
18
intensity level is 7.1~ that corresponds to a fiber mode diameter of
4.9 llm. Fig.S illustrates radiation patterns of the optical
waveguide after the thermal diffusion. It can be seen that the
width of the radiation pattern across one-half intensity level has
S decreased to 0.80 that corresponds to a mode diameter of 43 llm.
Therefore, the use of the present inventions has allowed a
mode diameter to be increased from 4.9 ~lm to 43 ~lm. In contrast
to the closest prior art, the mode field diameter conversion fiber
can be more easily fabricated by virtue of the fact that a number
10 of required process steps is decreased. Owing to the auxiliary
optical waveguide 7 spliced with the optical waveguide I subjected
to the thermal diffusion, the optical waveguide deformations,
occurring in the closest prior art owing to relaxation of stresses,
are entirely avoided in the thermal diffusion. Unlike the closest
ls prior art, the refractive index profile of a mode field diameter
conversion fiber produced in accordance with present invention
does not contain regions with a decreased refractive index, which
makes it completely compatible with the conventional ones.
Long-period gratings were also written in a quartz optical
20 waveguide I with a core 3 of Si3N4: SiO2, diameter 4.5 ~lm, and a
cladding 2 of SiO2, diameter 1 2011m, wherein the refractive index
difference between the core and the cladding was ~n = 0.012, and
the nitrogen concentration was I at. %.
In a first embodiment, a heating source was an electric arc
25 discharge 10 (Fig.6). The arc 10 was axially displaced along the
optical waveguide I by a translator 6 at a distance of 250 ~lm
controlled with an accuracy of 10 ~m. A distance between
electrodes S and current of the arc 10 were 150 ,um and 5 mA,
respectively, so that the size of the region 20, in which the
30 refractive index changed, was about the same as the diameter of
the optical waveguide 1. A microscope was used to monitor this
size by a characteristic dimension of the core 3 light emission
region measuring about 120 llm. The time it took for every groove
CA 02242842 1998-07-10
19
to be written was 1 s. The grating period was 250 ~lm, the total
number of grooves was 80.
Transmission spectrum (Fig.8) was monitored by an optical
spectrum analyzer 22. The spectrum shows seven absorption peaks
5 corresponding to resonant coupling of the core 3 mode to the
cladding 2 axially symmetric modes having dissimilar lateral
indices. This is unambiguously indicative of a periodic modulation
in the refractive index of the core 3 of the optical waveguide I and
a generation of a long-period grating of the refractive index of the
lo core 3.
To produce a preform for an optical waveguide I with an
undoped quartz glass cladding 2, a nitrogen-doped glass core 3,
and ~n = 0.02, a substrate quartz tube 24, having a diameter of 20
mm and a wall thickness of 2 mm, is heated to the temperature of
1300~C. A mixture 25 of composition SiCl4 + 02 + N2 is supplied to
the substrate tube 24 under a total pressure of 1 mm Hg.
Microwave power 27 is varied in the range between 1 kW and 5
kW. The required level of doping the glass with nitrogen is
obtained at mass flow ratios [SiCl4]:[02] = 0.92 and [02]: [N2] =
20 0.5. The amounts of oxygen and silicon atoms incoming to the
reactor in a unit time are in the ratio of 2.2, while the nitrogen-to-
oxygen atoms ratio is 2.
To produce a preform for an optical waveguide 1 with an
undoped quartz glass cladding 2, a nitrogen-doped glass core 3
25 and ~n = 0.01, a substrate tube 24 having a diameter of 20 mm
and a wall thickness of 2 mm is heated to the temperature of
1300~C. A mixture 25 of composition SiCl4 + 02 + N2 is supplied to
the substrate tube 24 under a total pressure of 1 mm Hg.
Microwave power 27 is varied in the range between 1 and 5 kW.
30 The required level of doping of the glass with nitrogen is obtained
at mass flow ratios [SiCl4];[02] = 0.92 and [02]: [N2] = 0.5. The
amounts of oxygen and silicon atoms incoming to the reactor at a
unit time are in the ratio of 2.6, while the nitrogen-to-oxygen
atoms ratio is 2.
CA 02242842 1998-07-10
6. Industrial Applicability
The present invention relates to fiber optics and can be
employed in fiber radiation collimators, optical waveguide
5 couplers, spectral filters, optical isolators, long-period gratings,
dispersion compensators, cascade mode field diameter conversion
fibers on simulated Raman effect, sensors of physical values
(refractive index, temperatures, stresses, etc.), radiation
suppression units for predetermined wavelengths, e.g. for
lo enhanced spontaneous luminescence peak at a wavelength of 1.53
~lm in erbium fiber amplifiers, spontaneous luminescence over the
region of 1.06 llm in neodymium amplifiers and lasers operating in
other spectral ranges, and for smoothing the gain spectrum in
erbium fiber amplifiers employed, in particular, in systems with
15 wavelength division multiplexing.
